1. Field of the Invention
This invention relates to a semiconductor photocathode emitting photoelectrons in response the incidence of photons and a photoelectric tube using the same.
2. Related Background Art
In general, the detectable longer wavelength limit of a semiconductor photocathode is largely determined by the semiconductor energy band gap of the light absorbing layer. For example, the upper detectable limit of wavelength is about 1.7 μm in a crystal system that lattice matches to an InP substrate.
On the other hand, a conventional photocathode is known that has a step-graded buffer layer on an InP substrate, wherein As to P composition ratio is gradually changed in the buffer layer (Japanese patent application laid-open No. 11-297191).
In this photocathode, light within the infrared region extending to a wavelength of about 2.3 μm is detectable, because the lattice mismatch between the InP substrate and the InGaAs light absorbing layer is reduced, the InGaAs light absorbing layer having In composition of 0.53 or more that is inherently lattice mismatched with the InP substrate.
A photocathode is also known (U.S. Pat. No. 3,958,143) where GaAs or GaSb is used as the substrate and various material systems, are used as the light absorbing layer. In this photocathode, when, for example, GaSb is used as the substrate and GaInAsSb having lattice constant closer to that of the substrate is used as the light absorbing layer, light having 1.77 μm wavelengh that is within the infrared region can be detected.